It is necessary to have an extra back up battery. The best external batteries are juice pack from mophie ($99.95). Please note: this one is only for first generation iPhone, the iPhone 3G compatible battery will be released in September. Mophie juice pack provides more power (1800mAH) than the iPhone internal battery (1400mAH).

The battery I raved about earlier in this thread has now 1770 mAh power and new electronics in it, so I ordered a new one. Here are the results:

PLUS

* I drained my iPhone to 0% energy and then plugged in the 3GJUICE to see if it would charge to 100%. It did. With the remaining energy, I filled a Nano.
Then it was exhausted, the poor thing. Pretty good..! Compared to the Solo, it's 100% more power in real life. Compared to the old Mophie: both are in the same league. The Mophie loads a little faster, but give less power then the 3GJuice.

* Refilling took 3 hrs (through USB-port).

* It matches my phone, same black and silver.

MINUS

* No manual, has to be download from site (green company. lol)

* Indicator is too simple for me: just green and red. Would like to see also orange

Agreed, if only to feel secure. I generally carried a back up battery for my Treo too, but rarely needed it.

Originally Posted by wonderwall

The best external batteries are juice pack from mophie ($99.95). Please note: this one is only for first generation iPhone, the iPhone 3G compatible battery will be released in September. Mophie juice pack provides more power (1800mAH) than the iPhone internal battery (1400mAH).

I got the Tekkeon MP1550 and it works great. The thing that had me buy this was that it takes rechargeable AA batteries and has a built in battery charger. That way in case of emergency, I can buy some AA batteries and charge the phone, but also charge the batteries for a digital cammera, etc.

Lastly, the capacity on NMHD AA batteries is around 2900 mah now which is bigger than most of the iphone specific chargers I have seen.

I bought two, one for a gift, on Amazon. Does not include the batteries. I paid $20- for four (2000 mah) nickel metal hydride batteries at Radio Shack. Cheaper batteries with lower capacity were available. So, this solution is about 20% cheaper than the Richard Solo Battery and has greater capacity. However, it is much larger. I would carry it in a brief case but not a pocket.

I carry my Richard Solo battery in the change pocket of jacket or trousers. I used it to recharge my iPhone in about an hour yesterday.

I found this 3G iPhone backup battery and compared with the market available 3G iPhone backup battery. This one is very powerful. I bought one called IMP500 and it is more powerful than I thought. After fully charge it up and it can be used to recharge 3G iPhone 4 times. I like to recommend it to 3G iPhone and 2nd Generation fan

I found this 3G iPhone backup battery and compared with the market available 3G iPhone backup battery. This one is very powerful. I bought one called IMP500 and it is more powerful than I thought. After fully charge it up and it can be used to recharge 3G iPhone 4 times. I like to recommend it to 3G iPhone and 2nd Generation fan

Pros

Big capacity,
Nice design
Easy to use

Cons

Need a USB cable, I like cableless more

But with 50$, it is still a good buy, check this out

imaxpower.com

When I googled this, the first url returned was to a video demonstration. 5000 mah. Same technology and source as the iPhone battery.

Big advantage of the USB cable is that the battery can be used to charge arbitrary devices.